TiN/Ti CMP converts Ti and TiN surfaces into TiOx passivation layers, enabling uniform mechanical removal of excess material while preserving the barrier layer .
After the bulk tungsten (W) deposition and subsequent W CMP step, the W overburden is removed, leaving the underlying Ti/TiN barrier la
yer exposed on the field dielectric . The Ti/TiN stack serves as an essential diffusion barrier and adhesion promoter between the W plug and the interlayer dielectric (ILD) . The TiN/Ti CMP step is required to completely remove this remaining field barrier layer, physically and electrically isolating the individual W contacts before the subsequent Metal 0 processes . This isolation is critical because any continuous residual metallic barrier would act as a parallel conductive path, causing catastrophic electrical shorting across the Middle-of-Line (MOL) layer (Engineering Practice). The TiN/Ti CMP process operates via a coupled "chemical passivation–mechanical removal" mechanism . First, oxidizers (such as H2O2) in the slurry drive surface electrochemical reactions that convert the hard metallic Ti and TiN surfaces into nanoscale metal-oxide passivation layers, such as TiOx . This chemical softening significantly reduces the mechanical strength of the surface layer compared to the bulk metal . Subsequently, nanoscale abrasive particles suspended in the slurry exert mechanical shear and normal stress—governed by Preston's law—to scrape away this softened oxide layer . The dynamic balance between the surface oxidation kinetics and the mechanical abrasion rate strictly determines the overall material removal rate (MRR) . Slurry design for barrier CMP requires precisely tuning the removal rate selectivity among W, TiN, Ti, and the underlying TEOS dielectric . Because the W plug is already exposed during this step, the slurry must feature a high TiN/Ti removal rate while suppressing further W etching to prevent severe W recess, also known as dishing . To achieve this balance, chemical inhibitors or multidentate chelating agents are often introduced to regulate the passivation film's stability and dissolution kinetics . Abrasive concentration and polishing pressure are then modulated to provide sufficient mechanical force to clear the barrier without inducing non-removable micro-scratches or severe erosion in the underlying dielectric . Furthermore, the slurry pH is strictly controlled to manage the double-layer electrostatic interactions—based on the isoelectric points of the materials—between the abrasives and the wafer, thereby minimizing particle adhesion and subsequent contamination . In 40nm BSI CMOS Image Sensors, the contact aspect ratio is high and the plug pitch is densely packed, necessitating exceptionally tight control over planarization efficiency and defectivity (Engineering Practice). Even minor W contact dishing or dielectric erosion can severely degrade the depth-of-focus margin for the subsequent Metal 0 lithography step . Consequently, ex-situ electrochemical optimization of the barrier slurry is often utilized to decouple chemical reactivity from complex mechanical interference, ensuring a predictable and stable integration window that satisfies the rigorous reliability requirements of the image sensor array .
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